Correct Tess every single time she starts to go for something your Cairn might have. EVERY. SINGLE. TIME. She'll pick up that it's not acceptable. If you cant be there to correct, someone needs to be crated so the behavior cant reinforce itself. Shasta does this with Dax. She gets corrected very quickly when she stands up to head towards Dax. She's gotten to the point she'll look to me to see if she can get away with it and all I have to do is shake my head no. She sulks but she's also almost 4 years old. Dax will try now and again, but even at a year old, he doesn't always try to get anything Shasta has. Shasta is the one I really have to watch. Unless there's a bone involved and then Dax is trying to steal that and gets busted on it.